1. Field of the Invention
The present invention relates to an optical frequency mixing apparatus for calculating a product value between a conversion signal of received intensity-modulated light and a predetermined frequency signal and outputting the product value.
2. Related Background Art
Light is often used for high-speed phenomenon measurement, high-precision measurement, and the like. This is because in principles, light is an electromagnetic wave having a very short wavelength, which can carry a large amount of information of target measurement phenomenon within a short time. In addition, progress in laser techniques, which allows light with good coherency and a high intensity to be obtained, also motives the widespread and actual use of light. On the other hand, most measurement apparatuses use an electrical method. Generally, electrical control can be most precisely and easily realized.
However, conventional electronic circuit parts are often so poor in response performance to directly convert information carried by light to be measured (i.e., waveform of the light) into an electrical signal. For this reason, it is necessary to convert the information carried by the light to be measured into a signal form which can be precisely controlled by an electronic circuit while preserving the information carried by the light to be measured.
In a typical signal conversion system, light to be measured and appropriate known reference light are caused to interfere with each other, and the light is converted into an electrical signal matching a frequency according to the difference between the two light wavelengths. The first example of this system is used by, e.g., a laser Doppler velocimeter, in which illumination light and light reflected by an object are caused to interfere with each other, and a beat frequency is measured. The second example of this system is a heterodyne interferometer. In these systems, two light waves having different frequencies are caused to be incident on the photoelectric conversion surface of a photodetector while keeping spatial coherency, and an electrical signal matching a frequency according to the difference between the wavelengths of the two light waves obtained from the photodetector is measured.
In recent years, the speed and precision of phenomena to-be-measured has become higher. Accordingly, as for a signal matching the frequency according to the difference between the wavelengths of the two light waves, measurement beyond the response characteristics of the electrical circuit at the subsequent stage is required. In some cases, light which is intensity-modulated within a known frequency range beyond the response characteristics of the electrical circuit at the subsequent stage is received. In this case, the characteristic frequency of the incident light must be further lowered.
FIG. 1 is a block diagram showing the arrangement of a conventional typical frequency mixing apparatus for lowering a frequency. As shown in FIG. 1, in this apparatus, light waves including the components of two frequencies (f1 and f2) are received by a photodetector (PD) 910. An electrical signal generated in the photodetector 910 upon reception of the light is supplied to an amplifier 920 and amplified. Thereafter, only a signal component having a frequency of about .vertline.f1-f2.vertline. is selected by a bandpass filter 930. The electrical signal passing through the bandpass filter 930 is mixed with an electrical signal having a frequency f3 in a signal mixer 940 and converted into an electrical signal according to the product value of the two signals. The signal obtained upon conversion includes the components of two frequencies (.vertline..vertline.f1-f2.vertline.-f3.vertline. and .vertline.f1-f2.vertline.+f3). A bandpass filter 950 selects the component of the frequency (.vertline..vertline.f1-f2.vertline.-f3.vertline.) and outputs this frequency component.
An apparatus is proposed in Japanese Patent Laid-Open No. 62-279732, in which intensity-modulated light is received by an avalanche photodiode (to be referred to as an APD hereinafter) which is biased by a signal superposed with an AC component, thereby simultaneously performing photoelectric conversion and frequency mixing. FIG. 2 is a block diagram of this apparatus. In this apparatus, intensity-modulated light modulated by the modulation frequency f1 is received by an APD 960 which is biased by a signal having the frequency f3, which is superposed with a DC signal. Upon reception of the light, a current is generated in the APD 960, and a voltage signal including the components of two frequencies (.vertline.f1-f3.vertline. and f1+f3) is generated at a position P. A bandpass filter 970 selects the component of the frequency (.vertline.f1-f3.vertline.) and outputs this frequency component.
The conventional optical frequency mixing apparatuses with the above arrangements have the following problems.
In the conventional optical frequency mixing apparatus shown in FIG. 1, when the amplitudes of the two light waves incident on the photodetector 910 vary, the electrical operating points of the photodetector 910, the amplifier 920, and the signal mixer 940 change to make the detecting operation unstable. Additionally, in an environment with large background light (DC light or light including intensity-modulated components unrelated to measurement), the performance cannot be sufficiently exhibited because of saturation of photodetecting elements used in the photodetector 910 or the mixing/modulation characteristics of the circuit elements of the apparatus. Furthermore, to perform heterodyne detection between light waves, the light waves are converted into electrical signals by the photodetector 910 and thereafter mixed with each other. Since this operation requires a high-speed amplifier at the first stage, the number of circuit constituent elements increases. In some cases, the detection precision of the difference frequency is limited due to noise, drift, or offset of the circuit elements.
In the conventional optical frequency mixing apparatus as shown in FIG. 2, the DC bias voltage of the APD 960 is superposed with a high-frequency voltage, thereby directly obtaining a heterodyne signal from the photodetector. In this apparatus, a desired heterodyne signal can be easily extracted at a high conversion efficiency without using any electrical mixer with a large loss. However, an APD generally has voltage-to-current characteristics represented as an exponential function, as shown in FIG. 3. Therefore, application of a high-frequency voltage having a large amplitude for deep modulation provides no advantage against distortion in the extracted heterodyne signal. In addition, to ensure an avalanche multiplication gain, a bias voltage is required to be applied to an almost pole of a breakdown voltage value. For this reason, a temperature compensation circuit for a bias voltage is essential to control a breakdown voltage having characteristics sensitive to a temperature. Furthermore, when a bias voltage value (V.sub.1) is set at a high level to ensure a large multiplication gain of the APD and increase the photoelectric conversion gain, operating points vary due to DC background light or light including intensity-modulated components unrelated to measurement. Simultaneously, the APD itself tends to be saturated by an increase in average current (I.sub.0).